Embodiments of the present disclosure are related to wind turbines, and more particularly to methods and systems for shutting down a wind turbine.
Wind turbines typically operate in a narrow range of wind speeds. Moreover, wind turbines operate optimally in uniform wind conditions. Accordingly, it may not be desirable to operate a wind turbine during turbulence, excessively high wind speeds or very low wind speeds. In these conditions, the wind turbine is usually shut down. Wind turbines may also be shut down for routine or exceptional maintenance. Typically, to shut down the wind turbine, rotor blades of the wind turbine are brought to a feathered parking position from their operating position. In the feathered parking position, the rotor blades are positioned perpendicular to a wind direction such that one edge of the rotor blades is directed towards a tower of the wind turbine and the other edge of the rotor blades is directed away from the tower. In this position, aerodynamic forces from the wind are practically zero.
During the shutdown process, wind turbines often face issues due to the effects of aerodynamic thrust on the wind turbine. These issues may arise because during the normal operation, the wind often places a positive aerodynamic thrust on the wind turbine in a direction that is perpendicular to a plane of the wind turbine. Subsequently, when a shutdown command is issued, the rotor blades begin to pitch out towards the feathered position. Due to this variation in pitch angle and rotor speed of the rotor blades, the aerodynamic thrust placed on the wind turbine may decrease, which induces oscillations in the wind turbine. Further, as the rotor blades continue to pitch out, the rotor blades may experience a negative aerodynamic thrust. Accordingly, instead of pushing the wind turbine in a downwind direction, the negative aerodynamic thrust may place a pull on the wind turbine in the upwind direction. In this situation, if an upwind oscillation of the wind turbine is synchronized with the upwind pull of the wind turbine, the oscillations of the wind turbine may be aggravated. Consequently, the wind turbine may experience large structural loads, potentially causing wear and damage to the wind turbine.
Currently, various techniques are available to shut down a wind turbine, i.e., pitch the rotor blades from their operating position to the feathered parking position. One technique entails pitching the rotor blades from the operating position to the feathered parking position at a uniform rate. This technique, however, can lead to large vibrations in the fore-aft direction. Another technique, commonly referred to as a triple-pitch braking, is often utilized to prevent the large structural loads associated with shutting down the wind turbine. In the triple-pitch approach, the rotor blades are pitched from their operating position to the feathered parking position in three stages. In a first stage, the rotor blades are pitched at a fast rate for a particular interval of time, for example 1.5 seconds. Thereafter, during a second stage, the rotor blades are pitched at a slower speed for a second interval of time, for example 1.5 seconds. In addition, in a third stage, the pitching rate is once again increased, until the rotor blades reach the feathered position. Though this technique attempts to obviate the shortcomings of the uniform pitching technique, the triple pitch approach is based on a pre-defined pitching profile and an open-loop controlled approach. Particularly, the pitching rate and the time interval for each stage of the three stages is determined based on worst-case expected behavior. Therefore, implementation of the triple pitch approach to shut down the wind turbine may also result in a negative aerodynamic thrust on the wind turbine and the accompanying drawbacks.
In addition to these techniques, various closed-loop controller techniques have been employed to shut down the wind turbine. Moreover, these techniques also attempt to obviate the issues associated with shutting down the wind turbine. One such closed-loop technique is commonly referred to as a zero-acceleration approach. In this approach, the rotor blades are pitched towards the feathered position until the aerodynamic thrust on the wind turbine is reduced to zero. Thereafter, the system controls the pitch angle of the rotor blades such that the aerodynamic thrust remains zero until the tower has reached an equilibrium position. Subsequently, the rotor blades are pitched again towards the feathered position. Though this approach may aid in reducing excessive oscillations in the tower, this approach prolongs the shutdown time.